1. Field of the Invention
The invention relates to an ink supply device for supplying ink to an ink jet recording apparatus, and particularly to a novel ink supply device which is intended to stabilize the ink supply performance and suppress the influence of a change of environment.
2. Discussion of the Related Art
Conventionally, a known ink supply device used in an ink jet recording apparatus has a configuration wherein the entire internal space of an ink tank communicated with an ink jet head is charged with an ink absorber made of sponge or felt, which is a fibrous material, the ink absorber is previously impregnated with ink to retain the ink, and the ink is in the ink absorber is supplied to the ink jet head.
In an ink supply device of this type, however, ink is retained by the capillary force of the ink absorber. Therefore, the amount of available ink is about 40 to 60% of the content capacity of the ink tank at the most so that the utilization efficiency is fundamentally low. Moreovor, the attempt of prolonging the service life of the ink tank inevitably causes the ink tank to have a large size, thereby producing a technical problem in that the requirement for miniaturization is not fulfilled.
Furthermore, when the amount of ink which is to be retained by the ink absorber is decreased as a result of consumption of ink, the negative pressure acting on the ink contained in the ink absorber is increased. The increased negative pressure is apt to hinder the operation of supplying the ink to the ink jet head, thereby arising a fear that printing nozzles of the ink jet head conduct the ink ejection operation under the state where no ink is supplied to the head. As a result, another technical problem is produced in that air bubbles are liable to enter backward from the printing nozzles to cause image defects.
In order to solve these technical problems, ink supply devices have been proposed in which a sealed ink tank, having a non-variable or fixed internal volume, contains ink only and is communicated with capillary tubes each having one end that opens in the atmosphere (for example, Japanese Patent Unexamined National Publication No. Sho 59-500,609, and Japanese Patent Unexmined Publication No. Hei 1-148,559).
In an ink supply device of this type, when the negative pressure of the ink tank is increased as the ink in the ink tank is consumed, air is introduced into the ink tank through the capillary tubes. This allows the negative pressure to be maintained at a substantially constant level, so that the ink in the ink tank is stably supplied to the ink jet head.
When environment is changed so that, for example, the air in the upper space of the ink tank is expanded, the ink in the ink tank is moved into the capillary tubes. Consequently, a situation in which ink leaks from the ink jet head is effectively prevented from occurring.
Since the sealed ink tank is communicated with a cell through capillary tubes of a substantial length, however, such an ink supply device is complicated in configuration by, for example, the provision of capillary tubes which are required to be suitably bent. Even in the case where the negative pressure of the ink tank is increased as the ink in the ink tank is consumed, furthermore, the introduction of air bubbles through the capillary tubes is made more uncertain when the capillary tubes are clogged by dust or dried ink. As a results, a further technical problem is produced in that the negative pressure of the ink tank is liable to become unstable and it is difficult to stably supply the ink to the ink jet head.